A structure that prevents chips from contacting a machined surface of a workpiece and damaging the machined surface is being considered for cutting inserts, particularly, cutting inserts for a cut-off process and a grooving process. Japanese Unexamined Patent Publication No. 2010-99815 discloses a cutting insert in which a cutting edge part and a square-shaped rake surface oriented in a thickness direction orthogonal to a longitudinal direction and a width direction of a shank-like insert body are formed at an end part of the insert body in the longitudinal direction thereof, and band-like helical surfaces extending along the longitudinal direction are respectively formed at both end parts in a width direction of the rake surface. The chips can be sufficiently rounded and easily divided by these two helical surfaces. This allows for a smooth and sure chip removal, and also prevents the chips from contacting the machined surface and damaging the machined surface.
However, for example, when the foregoing cutting insert is used for a face grooving process for forming an annular groove on an end surface of a workpiece, a chip discharge direction is apt to shift laterally because a cutting speed on a right end side of a cutting edge and a cutting speed on a left end side of the cutting edge are different from each other. Further, due to the influence of the cutting speed difference between the right end side and the left end side of the cutting edge, a curl diameter of chips generated on the right end side and a curl diameter of chips generated on the left end side are different from each other. Therefore, the chips are discharged spirally, and consequently there is a risk of deterioration of chip discharge performance.